Tighten is_batched_event sub-event check

[tomerm-iguazio]

Summary

• is_batched_event previously classified any event whose body contained items with a .body attribute as batched. This caught non-event objects like mlrun's Response, which also exposes .body.
• Tightened the sub-event check to also require "event" in the class name (lowercased), preserving duck-typing for non-storey event-like objects while excluding Response-shaped payloads.
• Added unit tests covering the positive case (storey Event sub-events) and the negative case (Response-shaped sub-events).

Test plan

• tests/test_flow.py::test_is_batched_event_with_event_sub_events
• tests/test_flow.py::test_is_batched_event_excludes_non_event_body_objects
• Existing batched-event flows still classify correctly in CI

Part of ML-12706 effort.